Amplifying voltmeter with a logarithmic scale



NOV. 13, 1951 1 SELZ ET AL 2,575,073

AMPLIFYING VOLTMETER WITH A LOGARITHMIC SCALE f77 Wax/mum anode carre/1t) Nov. 13, 1951 1551.1 ET AL` 2,575,073

AMPLIFYING VOLTMETER WITH A LOGARITHMIC SCALE Filed May l2, 1948 2 SHEETS--SHEET 2 ,L//sg/ PES.

Patented Nov. 13, 1951 UNITED AMPLIFYING VLTMETFR WITH A LOGARITHMIC SCALE Jacques Selz and Alexandre de Leudeville, Paris, France ApplicationMay 12, 1948, Serial o; 26,634

In France May`13, 1947 4 Claims. i

In order to obtain suiciently precise readings when voltages to be measured vary within wide limits (for example, from a few microvolts to a few volts) without being subjected to any operation such as is required for the recording of the readings, it has been proposed to provide a voltmeter with a logarithmic response curve. The relative precision of the apparatus thus produced is constant whatever may be the value of the voltage to be measured: the instrument dial carries a graduation which is preferably either in decibels or in Napiers.

Such apparatus has already been proposed, but the range of measurement appeared to be of insufficient extent, and the purpose of the present invention is to supply for the engineer an apparatus the principle of which allows adapting it without fundamental modification to any desired range of voltages.

This present invention therefore relates to a voltmeter with a logarithmic scale for the measurement upon a single scale, of alternating voltages varying within wide limits, characterised in that it comprises substantially a multi-stage amplifier constituted of several amplifier stages connected in series and to which is applied the alternating voltage to be measured, an individual liminal value detector connected to the output of each amplier stage, for deriving a portion of the current passing through the series of stages, and, at the output of each such liminal value detector, an individual direct current amplifier, formed by a tube of which the anode current-control grid voltage characteristic is logarithmic.

If it is a problem of measuring for example a range of voltages ranging between the values a and alan volts, the number of such amplifier stages will be equal to n and the amplification factor of each of them with the possible exception of the first one will be equal to lc.

To simplify the description which follows, it will be assumed that the range of voltages to be measured is represented by the interval from a to 10,0Ga microvolts, that is, 80 decibels: there will be four separate alternating current amplifier stages each with an individual gain of 20 decibels, except for the initial input amplifier stage, the gain of which will be specified later. Thus, the range to be measured is from the value a to a value aku equal to a .104.

At the output of each alternating current amplifier stage there is provided according to this invention a direct current detector-amplifier system; each such direct current detector-amplifier system supplies direct current varying as a funcl tion of the peak value of the alternating voltage applied thereto and is arranged in such manner that:

(l) Below a certain liminal value 111 of the peak alternating Voltage applied thereto, the output direct current I of the detector-amplifier of that stage is constant and equal to a certain maximum value IM.

(2) For the values of the peak alternating voltage comprised between the valve 'U1 and a higher value 'U2 the output direct current I decreases from IM to a zero value.

Thus there are four units of amplier-detectors each of which individually delivers a derived direct current varying as hereinbefore mentioned. The four currents from these four separate amplifier detectors pass in the same direction, and are added in the input of the measuring-indicating or recording apparatus which may be of galvanorneter type.

The assembly of direct current detector-amplier units, is arranged in such manner that for any value of the applied alternating voltage to be measured taken in the measuring range, there is one given single detector-amplifier which is controlled by a peak value of the applied alternating voltage comprised between values of v1 and v2 and therefore delivers a direct current I whose value is between the values 0 and IM, the detectors more remote from the point of application of the voltage to be measured, delivering a negligible current, and those nearer the point of application of the voltage to be measured supplying a direct current whose value is equal to IM.

The designation active amplifier will be given to an amplifier unit delivering an output direct current whose value is between the values 0 and IM: when the current delivered is negligible, the amplifier unit will be termedfsaturated, and when this current is equal to IM the amplifier unit will be termed inert For each value of the voltage to be measured or to be recorded, lower than or equal to the value a microvolts, the four direct current amplifiers are inert, the galvanometer or indicating instrument therefore receives current of the value of 41M.

For an applied alternating voltage V comprised between a and 10a the fourth detector is active owing to the gain of the four alternating current amplifiers, and. the indicating instrument G is traversed by an output current SIM-l-I this current becomes 31M when the voltage V becomes equal to 10a.

When the applied voltage V is between 10a and 100a, the fourth direct current amplifier unit is saturated, while the third is active owing to the gain of the rst three alternating current ampliers; the indicating instrument therefore receives current of the Value of 2IM+I which becomes ZIM when the applied voltage V=la.

Whenthe voltageV is .betweenlOOaand 1000e, the two final directcurrent amplifier units are saturated and the second is active, the instrument therefore receives current of the value of IM-l-I, and finally receives current. of .the .value IM when the applied voltage V is equaltto y100011.

Finally when the applied Voltage V is between 1000a and 10,000a, the three final direct current amplifier units are saturated, and the first is -active, the indicating instrumentis` thereforetraversed by current of a value I which becomesfzero when the voltage V is equal to 10,000a.

To sum up, the scale reading or deflection of the galvanometer instrument varies by quantities which are-edualfor'values of applied'voltages which. vary in equal' ratios, :that is,::inigeometrioal progression. The response of the apparatus-is exactly logarithmic lfor: the ia'ppliedavoltage values a; a,i 1:00a ,and-so. forth which-correspond ,to 'the application' of the: respective liminal'fvaluesiof the actual inputs of thevar ious detectors.

In the intervals between these specific values, deecticns will be produced of 'values related approximately logaritbmically by verriplyirig as direct-current :amplifiers according -;to 4,this present invention, variable-muvacuumrtubesgthe anode current-control.- gri'd voltage` .characteristic ofwhich.-is close'toia logarithmic curvefor constant-value of: anode voltage. =Itfollows-from this that a scale-graduation reading in decibels -between'O and`20,or rbetween 20-,and'40iand sogon, will `present fairly equal divisions.

:In reality the characteristics vof thevariable mu vacuum tubes are not logarithmic over A:their whole extent, but have a curvature in; .one direction 4for the current I below ya veryismallzvalue close toIM, and a curvature inthe.r otherdirection for Ii. In-practice, for an appliedfvoltage close -tc the values '1011, il00a v'or 1.00.0015, two detector-amplier units will deliver` the current. By the combined effect of the twocurvesof` opposite curvature. the variation ofthacurrentremains logarithmic `for these values of..voltages. Thereresults-a scale in decibels whichisza'lmost exactly linear except,` it will beappreciated,.'for the limit values a and 10,000a forwhich thisconipensationiiefnct produced, which causes slight irregularities at the startl and end `ofrtlie scale.

Ithas been stated above-thattheinitial alternating current-input-amplifier stage' has a. gain different fromdb. In fact, in the frequency band considered, this input ampliiier stagegives the gain necessary to obtain proper operation for the 'maximum Voltage 10,000a'to be measured or to be recorded (l0,000a)to correspond'to the reduction to substan'oially'zero`r from .the value 4TMfof the total output directcurrent, thewvalue 41M corresponding to .an applied voltage equal to orfless than thelowest .valuefapffthea-voltage range to be-rrieasure'dI or recorded. Avoltageften times weaker than 10,000a, that is, l000a, should attain lthe liminal level of lthe first detectonithat is,-it' should apply to-this detector a;value--Which is justV high enoughrto make this detectorstart to become active. y

If the valueofthe .voltage at the'bottomcf the scale a is taken .forinstance -as equal'to 10 microvolts (effective) then there :will'bein this case 1000e-:10.01 eiective 1volts5appliediat effective volts.

In reality, to take into account the voltage eiiiciency of rectification of this dei tectoiiassumedjto be close to 0.7, that is, the ratio of the `-.direct current Voltage at the output of .thedefector to the peak input voltage of the de- .tector,'this voltage must be equal to 0.5 eective volts. rEhe initial alternating current input amvpiiiierzstage tmust-.therefore give voltage an amlplilication equal to that is, a-gain-invdecibels of about OLE-1:34 decibelsv Y This ratio of 50 ywhose logarithm is taken is determined vas the ratio of the desired liminal value ofthe input alternating voltage o'f the 'detector`0l5 vclt, to the assumed effective value 0:01 volt at. the input of theV initial alternating current amplifier.v

Z'Ingeneral, the gain of the rst initialalternating current amplier stage will' be represented by (20e-Y) db,-:whereiY ris-a 'difference-factor determined inthe mannerl just. explained.

The :arrangement` of the apparatus will -now be described foran applied voltage interval of db. with reference to the accompanying drawings: wherein:

Fig. l is a block diagram showing thegeneral plan of the-arrangement of the Variousparts of thezvoltmeter system.

lig. 2 is the circuit arrangement of a liminal detector with its associated directcurrent'amplifier.

Figs. 3a, 3b, Scare graphs of'the principal characteristics of the system.

Fig. 4vis the circuit of a voltage limiter serving vas a-safety device.

Finally' Fig. 5-shows a planof switching circuit a'llowingoffincreasing the precision ofthe vcltmeter used as indicator.

Asshown with're'ference to Fig." 1, |0,20, 30 'and l0 vare the alternating current amplifier stages and Il, 2|, 3l and 4l are'y the liminal detectors. l2,"22,32. and 42 are the direct-current amplifiers. 23 and 33.are the limiters. G is the dial of the galvanometer used Yas indicator.

The initial :amplifier stage IB has .again of @O+-Y) decibels when :Y is` determined-as above indicated, each of the amplifier stages I20, y30 and'li has a gain of '20 db.

As shown in'Fig..2 each liminaldetector Il, 2l,3l,..4l comprise a diode I thecathode 3 of which is brought to fixed positive biast, for example ;t='l-.0..5 v olt. *The anode 2 of each diodel is connected through a: condenser andan inputterminal J ltorthe outputV of an alternating current amplifier, as liminal detector I l isiconnected to the output of amplifier .illLin,Fig. `1. Each direct current amplifier l2, 22,32, 42 is constituted by a-variable mu tube 5 theplate 1 of which ',isccnnected to-the measuringrgalvanometer G. The cathode of the variablemuatube ofthe .direct current amplifier is given afixed bias The negative .bias developed '..bythe passage of the rectified current through the resistance 4, across the anode 2 of the diode I, is transmitted through the lter network consisting of resistance r and condenser C, to the grid 5 of the tube 5.

The anode current Ia of the direct current amplifier tube 5 varies as a function of the negative bias of the grid as shown in Fig. 3a for constant plate voltage.

The direct current output of an amplier such as I2 (Fig. 1) is a logarithmic function of the input excitation voltage Vm applied to it, as shown in Fig. 3b.

The total current, that is, the sum of the four partial currents from the four direct' current ampliers I2, 22, 32, 42, passing through the galvanometer With substantially linear characteristics shown in Fig. 3c is in semi-logarithmic ordinates. Y

The outputs of the four direct current amplifler'stages are connected with a single Wire which passes through the galvanometer G, which thus indicates the sum I of the four individual currents I1, I2, I3, and I4, at the output of the four direct current amplifier stages, respeetively. The values of these currents, with the exception of one of them, is either IM or zero, as has been previously explained. When plotted on the semi-logarithmic scale of Fig. 3a, the total current I passing through the galvanometer G is a straight line, as shown in this Figure 3c.

The duty of the limiters 23 33 is to prevent the alternating current amplifiers placed after them as 35, lill from being saturated when those placed before them take care of the maximum output voltage provided for. These limiters may be of any known type.

These limiters 23, 33 may be constituted in particular as shown in Fig. 4 by a vacuum tube 4I the plate of which is connected to an impedance z and the grid I2 of which is connected to earth through a high resistance R, and the cathode 43 of which is given a xed bias h determined by the operating conditions of the amplier. When the alternating voltage applied to the grid exceeds h as the peak value, the grid current brings about in the resistance R a voltage drop causing a negative bias of this grid 42, the amplication is reduced and the limiter transmits through thecondenser C to the alternating current amplifier stage placed before it, (say the limiter 23 to the amplifier 35 for instance, Fig. 1,) a voltage which remains xed at a peak Value about equal to h.

No limiter is needed ahead of the amplifier I0 Fig, 1, because for the highest voltage to be measured, the amplifier IIJ furnishes a peak voltage of h volts which does not exceed the saturation limit of the devices placed immediately ahead of amplifier Ill.

In the apparatus described the galvanometer deflections are reduced when the applied voltage increases. If it be desired to obtain deections in the usual order of increasing values there will be applied to the galvanometer a constant compensation current equal to 41M from which will be substracted the current I=I1+I2II3|I4.

In the preceding description it is assumed that the range of measurement extends over 80 decibels: if 160 points be marked on the dial of the apparatus, each will represent a half decibel.

According to another characteristic of this present invention, precision may be increased in the case in which the apparatus is used as an indicator and not as a recorder. For that purpose use is made of the circuit shown in Fig. 5.

As shown in Fig. 5 there are inserted between the detectors and the direct current amplifiers, the rotary selecting switches 5I, 52, 53, 54, 55, 56, 51, and 58 each with 5 positions a, b, c, d, and e. These switches are connected mechanically to one drive shaft in such manner that al1 the sliding contacts are oriented invariably at the same position, the contacts of the different positions of the rotary switches being connected electrically together as shown in the Figure 5. The detectors are designated by numerals 63 to 65 and the direct current ampliers by 59 to 62. The arrangement allows of applying the grids of the tubes of 59 to 52 to the detectors I3 to 6E, -or in parallel across the given detector.

Assume that it is desired, for instance, to measure the voltage level 34.7 decibels. Then the rotary switches all being on position a a reading is effected under the same conditions as explained above; thus a value comprised between 34.5 and 35 decibels is read ofi on the dial. Then the rotary switches are placed on the position d, that is, the grids of the four variable mu tubes are placed in parallel across the anode of the active detector, 55 in this case. Then the total deflection of the dial of instrument G takes place for 20 decibels instead of for 80 decibels; if a second scale corresponding, and marked with 200 points, is provided on the dial then a tenth of a decibel can thus be measured. Thus in the case of a voltage of 34,7 decibels to be measured, the value 14.7 will be read on the scale, which gives for the level, the true value 20+14.'7=34.7 decibels.

The entire specic description given above to explain the constitution and manner of operation of the apparatus is based as stated upon a measurement range of decibels; it is evident that for other values of Ic and of n, the number of alternating current amplifier stage and of direct detector-amplifiers will be modified consequently as well as the gains of the amplifier stages.

In general, the ratio 7c is determined by conferring upon the amplifier stages 2D, 30, and so forth, on the one hand an amplification factor of 7c and on the other hand on the bias of the lminal detector shown in Fig. 2, a value equal to o/ (lc-1) 1J being the absolute Value of the variation of grid voltage causing the current of the direct current amplifier to pass from IM to a very small value i as shown in Fig. 3a. In fact the output plate current passes from the maximum value IM to a negligible value i for alternating current peak voltages applied to the detector varying from value V1 to value V2 such that the current starts to decrease for a value V2=v.

It reaches the negligible value i for Therefore t='U/(7c-1) where t is the xed positive bias of the liminal detector.

In all cases the principle of the circuit arrangement and manner of operation of the apparatus remain identical.

In the case of change in the amplification tubes, the characteristics of the assembly of the apparatus may be retained through a particular regulation provided on the screen and the grid of each tube.

afs-moza I-t-fwillbe apparent` tol those skilled in the ,art that our invention ris susceptible of modifications `to-adapt the same to particular applica-tions, and Aallsuc'h modifcations which are within kthe scope .of the appended claims we consider to,` be comprehended within the spirit or" our invention.

What we claim is;

1. A vol'tmeter with logarithmic scale vfor the measurement of palternatingvoltages lcomprising an input `alternating -current amplier to which the voltage to 'bey measured is impartedyfurther .alternating currentamplifiers having equal gains and arranged in series and in series with the yi'irst mentioned amplier; identical units each -of which comprises a threshold detector and a `direct current ampliiier, one of said units being arranged in shunt at the output of each of said alternating current amplifiers including the input amplier; the output rectied voltage of each threshold detector constituting the negative polarization voltage of the direct current amplier associated with said detector; the direct current ampliers having a characteristic curve anode current-log. control grid Voltage comprising a sloping portion the larger part 4of which follows a rectilinear course, the sum of said anode currents varying itself in proportion to the logarithm of the voltage to be measured; a common circuit for feeding the anodes of the direct current ampliers; and a galvanometer arranged in said circuit.

-2. In a logarithmic scale vacuum tube voltlmeter system, a plurality of amplifier Vstages connected in series, Yan input connection connected to the input of an initial said stage at the head of said series, the gains of all of said stages ex- .cept said initial stage having `the same predetermined value, an individual liminal detector connected to the Youtput of each said stage, an individual directv current amplier for each said detector having its input connected to the output of each said detector, and a galvanometer having a logarithmic scale connected to the outputs in parallel of all said direct current ampliers, said detectors respectively havingthe same control liminal values, each said direct current amplier comprising a variable mu tube having an anode current against control grid voltage characteristic which is substantially logarithmic.

3. A system -according to claim 1, each said detector and the direct current amplifier connected thereto constituting an individual control unit, and each said control unit comprising a variable mu tube having an anode current against control grid voltage characteristic which is substantially logarithmic.

4. A system according to claim 2, and a voltage limiting device inserted in series between the input of the last said amplifier stage and the output of the immediately preceding said ampliiier stage.

JACQUES SELZ. ALEXANDRE DE LEUDEVILLE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,406,882 kYoung Sept. 3, 1946 2,496,551 Lawson et a1. V l"eb. 7, 1950 

